Art dealer Alexander Acevedo was offered two different paintings at his Upper East Side gallery one morning last week. The prices were attractively low, he said, but something was wrong. It turns out the works were fakes.

The first was a poor quality impressionist painting with the forged signature of 19th-century French artist Berthe Morisot; the second was an unsigned Western scene, which the seller claimed was a work by a 19th-century American painter, Albert Bierstadt.

“I’m bombarded with fakes,” Mr. Acevedo said. “It goes on every single day. It’s like a minefield. Wherever you turn there’s a fake out there.”

The Alexander Gallery on East 74th Street and Madison Avenue, which Mr. Acevedo has run for the last three decades, specializes in American art of the 19th and 20th centuries. Another gallery with the same specialty, the Hirschl & Adler Gallery, has been seeing an increasing number of fakes and forgeries, the gallery’s president and director, Stuart Feld, said. “I’ve been seeing a huge number of fakes,” he said. “We have hardly a week that goes by that we don’t get two or three such submissions.”

As the market for art spirals higher, especially works of American art, the number of forgeries and fakes is also on the rise, art industry experts and dealers said. The origin of the increasing number of fakes is a mystery and leads to much speculation by many in the art world that everything from international conspiracies to greedy art teachers are in on the trade. Those who have been caught include convicted art dealer Ely Sakhai and amateur artists.

One week an artist will be featured on the front page of Art News or Art + Auction, and soon after something fake will surface, the former director of the Metropolitan Museum of Art, Thomas Hoving, said. Usually it’s not something as simple as a copy of the work featured at an auction, but an unexpected, yet related piece. This could be, for instance, a sketch by the artist, a pre-study for a sculpture, or a painting from the early part of their career. The works don’t sell for an exorbitant amount, but like any scheme of underworld entrepreneurship they move quickly and feed off the economic heat of the legitimate market.

It’s the ambitious dealers who take the biggest hit because they are the ones who are most likely to take a risk on a mysterious work. Not only do they have to sustain the loss with little possibility of getting their money back, they sometimes become paranoid, Mr. Hoving said.

“A forgery is dead on arrival and it infects other things,” he said. “You begin to look at everything with suspicion. Real stuff begins to look like them.”

Some of the forged or fake pieces are able to infiltrate the legitimate art world through antiques dealers, small-town auctions, and occasionally the unscrupulous dealer who sells the work anyway. Sometimes these pieces get identified as legitimate work, a problem that hurts the quality of art in general, the curator for American art at Harvard University’s Fogg Museum, Theodore Stebbins, said.

“It is very, very bad for art because once forgeries are accepted, then it lowers the level of the quality of art,” he said. “If more and more forged Rembrandts are out there, they don’t look very good compared to a real Rembrandt. Pretty soon our feeling is that Rembrandt is not a good painter. It changes our view of the artist and in the end diminishes it. It’s potentially damaging to all of art, to people who care about art.”

When it comes to examining a painting, most dealers are equipped with little more than a black light and their most valuable analytical tool of all.

“It’s called an eye,” Mr. Feld said.

The skill is known as connoisseurship, and is acquired after working with art for a lifetime, lawyer and author of “The Expert versus the Object: Judging Fakes and False Attributions in the Visual Arts,” Ronald Spencer, said.

Mr. Feld, for instance, has been at work on the catalog raisonné — the complete index of an artist’s work — of Frederick Childe Hassam for the last three decades. If someone submits a work of art to him purporting to be a genuine Childe Hassam, whether it’s a print, watercolor, sketch, or any other medium, something will immediately click in his head, he said.

“After seeing thousands and thousands and thousands of paintings your body builds an instinct,” Mr. Acevedo said. “All these little things go off at one time: Danger, danger. The color, something’s wrong. It’s instinct. You go on that every time you buy a painting.”

Mr. Stebbins of the Fogg Museum is worried that this tactic of detecting fakes is relied on too heavily. He is giving a lecture on the subject at the International Foundation for Art Research in November entitled “The Growing Problem of Fakes and Forgeries in American Art.”

“The fakes are becoming so sophisticated that it’s becoming very hard to tell them from the eye,” he said. One feature he said he is seeing on a number of forgeries is an increased use of a restorer’s skills to make the painting look genuine.

One of the FBI’s special agents that investigate art crime, James Wynne, said investigating and prosecuting art forgeries is difficult because to charge mail or wire fraud you have to prove a seller’s intent to deceive the buyer.

“It’s a big problem, the sale of fake art,” he said. “It’s not illegal for someone to create a painting and sign Chagall on it. The violation comes in the sale – the misrepresentation of the painting as an original.”

The New York Police Department’s detective who investigates art crime, Mark Fishstein, said many art sellers believe they have a genuine work of art.

“A lot of time they have work by the students of the artist,” he said. “And some painters didn’t sign their work,” making it hard to verify the work’s provenance.

And because many dealers don’t want openly to advertise the fact that they were duped into buying a misattributed or forged painting, the rising tide of fakes isn’t likely to be stemmed by law enforcement, dealers said.

“You need 1,000 to 1,500 Jim Wynnes,” Mr. Acevedo said. “It’s tidal wave proportions.